1. Field of the Invention
This invention relates to metallurgical processing and equipment and more particularly to equipment and processing for recovering metallic copper from copper bearing solutions.
2. Description of the Prior Art
In the production of copper it is a conventional practice to leach copper ore with sulfuric acid. This practice produces a solution containing dilute copper sulfate. To recover metallic copper from the dilute copper sulfate solution, it is also a conventional practice to precipitate the copper metal out from the solution using scrap iron for the precipitant. The process of precipitating copper metal from copper bearing solutions using scrap iron is generally called "cementation" and the precipitate is called "cement copper".
In the process of copper cementation there are three important chemical reactions. These reactions are:
Fe(metal) + CuSO.sub.4 .fwdarw. Cu(metal) + FeSO.sub.4 [ 1]
Fe(metal) + Fe.sub.2 (SO.sub.4).sub.3 .fwdarw. 3 FeSO.sub.4 [ 2]
Fe(metal) + H.sub.2 SO.sub.4 .fwdarw. H.sub.2 + FeSO.sub.4 [ 3]
It should be noted that in each of these reactions iron (Fe) is consumed. Reaction number 1 is the basic copper cementation reaction wherein the metallic copper is deposited on the surface of the iron as the reaction proceeds. Reaction number 2 depends on the amount of ferric iron present and usually consumes very little of the iron present. Reaction number 3 is the major iron dissipative reaction and does not contribute to the production of copper metal.
The oldest, most common apparatus for copper cementation is a gravity-flow launder that is charged with scrap iron and maintained in a flooded bed during processing. Although launder plants can effectively recover approximately 90% of the copper from copper bearing solution, launders fail to effectively stir up the bed of scrap iron, consume from two to four times the amount of iron theoretically required, and are difficult to recharge when depleted. Another prior art copper cementation device uses a rotating drum precipitator that continuously tumbles the total mass of scrap iron in the copper bearing solution. This requires excessively high capital and operating costs for the machinery involved. Other systems have been proposed, but they have either been too costly to implement or lacked the ability to obtain an adequately agitated bed and good precipitate-precipitant contact. An example of such systems included U.S. Pat. No. 3,606,290 to N. L. Ransom. One element common to the prior art processes is the continuous maintenance of the total scrap iron beds in a flooded or immersed condition with withdrawal of the barren solution as overflow. This continuous exposure of the entire bed of iron to sulfuric acid causes excessive consumption of iron by reaction [3] above which does not contribute to copper production. There is therefore a need for an improved copper cementation system with a reduced rate of iron consumption.